The Zojirushi Tuff Mug is made of durable stainless steel and is a little bit shorter and squatter than the Zojirushi Tuff Slim. The lid is designed to fit tightly in the extra wide mouth and has a storage space built in that can hold a packet of sweetener or a tea bag or two. Like all of the better travel mugs, the Zojirushi is vacuum insulated for extra long heat and cold retention. The stainless steel construction makes this mug durable and the lid is reportedly spill proof.